Introduction to the giant tortoise: The Galapagos giant tortoise is the largest existing tortoise and the longest-lived animal on earth. It is unique to the archipelago. An adult tortoise is 1.2 meters long and weighs up to 300 kilograms. Each tortoise's shell has a different shape. Giant tortoises are herbivorous and often eat plants such as leaves, fruits, weeds and cacti. They rely entirely on rainwater and cacti for their water sources.